There is a growing desire to develop biodegradable film, especially such film suitable for packaging, and an increasing desire for packaging materials that can be made using biologically-derived and/or renewable sources. The current industry standard for biodegradable packaging film is Ecoflex® polyester, a commercial biodegradable copolyester based on 1,4-butanediol (“4G”), terephthalic acid (“T”), and adipic acid. Such copolyesters are sometimes referred to as “4GTX” where “X” is a linear aliphatic dicarboxylic acid, here, adipic acid. Starch is also blended with such polymers because it can increase the renewable content and degradability of the polymer and lower its cost.
Biologically sourced 1,4-butanediol is not currently readily available, but 1,3-propanediol (“3G”) is. As one alternative to 4GTX, biodegradable copolyesters “3GTX” can be prepared based on 3G, T, and an aliphatic dicarboxylic acid. However, it can be difficult to obtain properties suitable for film with 3GTX/starch. Material for packaging film requires high maximum melt draw during manufacture, and the resulting film has to have adequate mechanical properties, such as toughness. A film of a specific composition made on a lab scale may have adequate toughness, as indicated by the percentage elongation or the area under the stress-strain curve, but to manufacture a film requires an adequate melt strength (defined as maximum draw times melt tension) to permit melt blowing processing, to achieve molecular orientation in films, and to promote strain hardening (strain-induced crystallization) during film blowing.
It is difficult to make 3GTX directly with high enough inherent viscosity (IV) for film, so instead a lower IV copolymer can be made and then built up using a chain extender, e.g., 4,4′-diphenylmethane diisocyanate (MDI). However, this approach can be detrimental to the toughness of films of 3GTX blends. There remains a need for formulations suitable for films, preferably with biodegradable and/or renewable content, that are biodegradable, possess sufficient melt strength for processing such as film blowing, and produce films with desirable physical properties.